1. Field of the Invention
The present invention relates to an image reading device that scans images by moving a slider for use in copiers and image readers.
2. Description of the Related Art
In general, there are two drive methods for devices which scan images by moving a scanning unit. Japanese Laid-Open Patent Application No. 5-303147 discloses a unilateral drive system providing a main guide 41 disposed on one side of a main housing to support a slider 40 so as to be movable in the scanning direction, and having a drive wire 43 fixedly attached to a slider end 42 positioned on the exterior side of said slider 40 so as to drive said slider 40 by means of said drive wire 43, as shown in FIGS. 4(a) and 4(b). U.S. Pat. No. 4,355,883 discloses a bilateral drive system providing drive wires 31 and 32 respectively attached to the bilateral ends of a slider 30 so as to drive said slider 30 by means of said drive wires 31 and 32, as shown in FIG. 5.
The unilateral drive system uses only a single drive wire 43, and generates yawing due to the deflection of the drive force. In the unilateral drive system of FIGS. 4(a) and (b), a slide member 44 provided on slider 40 is presses against main guide 41 by a spring 45 to prevent the aforesaid yawing. This elastic pressing force, however, cannot adequately suppress the yawing generated by slider 40.
In the bilateral drive system, two drive wires 31 and 32 are provided in identical bilateral mechanical constructions so as to transmit the rotational force of the drive motor 33 as equal direct drive forces to both sides of slider 30. As shown in FIG. 5, however, a transmission mechanism 37 is provided on either right or left side at the action point for transmission of the drive force from drive motor 33 to the slider 30, such that the transmission of the drive force to the transmission mechanism 35 on the opposite side is accomplished via a drive shaft 34 connecting both transmission mechanisms. When restriction of the space for installation prevents using a thick drive shaft 34, elastic twisting is generated in the drive shaft 34, such that a temporal dislocation occurs between the two transmission mechanisms when driving, which readily produces yawing of slider 30. Furthermore, although drive shaft 34 is mainly driven in its lengthwise direction in FIG. 5, temporal dislocation of the drive of both transmission mechanisms when driving is produced by precision discrepancies between said bilateral mechanisms as well as low rigidity of the pulleys 35 through 38, thereby causing yawing of slider 30.